Major concepts

1. What type of molecule in the membrane structure creates the barrier? What type of molecule performs most of the membrane functions?

Core knowledge

1. How do the faces of the bilayer differ from each other?

2. How are the types of integral membrane proteins different from each other?

Membrane functions
semipermeable barrier--really the result of the lipid core limiting movement of charged
     and polar molecules
communication with environment -- many molecules interact with membrane proteins ->
     changes within the cell, even though the molecules don't enter
regulation of cell function by controlling cytosol content
metabolic reactions occur in membranes (two-dimensional, not three-)

Membrane structure
A. phospholipid bilayer, with the exact composition of the phospholipids varying with
     species, tissues, organelle, etc.
      Contrast bilayer with micelle or liposome.
B. Membrane faces (monolayers):
      composition of one face ≠ composition of the other face
     1. phospholipids don't easily move from one side to the other
     2. protein composition varies from one side to the other

C. Proteins: perform many membrane functions
     1. peripheral proteins: loosely associated with the membrane, water-soluble
     2. integral proteins extend into/through the membrane
          a. are asymmetric in relation to the membrane
          b. types I & II have only one helix and differ in where (in or out) the NH₃-terminal is
               type III has several helices, one polypeptide
               type IV has several helices, one per polypeptide
               type V have lipid anchors; most are internal, but GPI-linked proteins are external
               type VI have transmembrane helices + lipid anchor(s)
                   some have carbohydrate residues attached – extracellular
                   example = GPI (glycosyl-phosphatidyl-inositol)
         c. composition: nonpolar (hydrophobic) amino acids span the membrane
                  α-helix: sequence of 20–25 nonpolar residues = one helical span
                   β-barrels: sequence of 7–9 nonpolar residues = one beta span
         d. hydropathy index (?G for transfer of the side chain from a nonpolar solvent to H₂O)
                   a good predictor of membrane-spanning helical segments, but not sheets

Membrane behavior = fluid mosaic
A. Proteins move within their layer (within their plane).
          Some proteins are anchored to internal cell structures and move very little.
B. Lipid behavior:
      1. Fatty acid chains move readily above a transition temperature characteristic of the
               membrane (determined by degree of saturation, amount of cholesterol, etc.).
     2. By adjusting phospholipid composition, cells maintain a consistent fluidity.
     3. Fatty acids also move within their plane (layer), quite rapidly.
     4. Flip-flops: catalyzed by flippases, required during membrane synthesis

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